1. Field of the Invention
The present invention relates to a wire electric discharge machine configured so that its machining state can be detected.
2. Description of the Related Art
In electric discharge machining, a voltage is applied to an electrode in a working fluid and a workpiece, thereby generating arc discharge. The moment the workpiece is melted by heat generated by the electric discharge, the working fluid is heated and explosively vaporizes, thereby blowing off melted portions of the workpiece. Machining is performed by frequently repeating this process. Since small discharge craters created by the electric discharge are collected to form a machined surface, moreover, the surface roughness depends on the size of each individual discharge crater.
Thus, it is known that a fine machined surface can be obtained by applying a high-frequency AC voltage (FIG. 1) to a machining gap and frequently repeating electric discharge of a short time duration. For example, Japanese Patent Application Laid-Open No. 61-260915 discloses how a machined surface with a surface roughness of 1 μm Rmax or less can be obtained by machining a workpiece with a high-frequency AC voltage of 1 to 5 MHz. If a high-frequency AC voltage is used for machining, however, it is difficult to precisely detect the machining state, and there is a lot of room for an improvement in machining accuracy.
In general, in electric discharge machining, an average of absolute values of machining-gap voltages is measured to determine the machining state, the electrode feed rate is controlled, and machining conditions are changed and controlled. The average machining-gap voltage practically represents a machining-gap distance, so that a high-precision machining shape can be obtained by performing electrode feed control such that the average machining-gap voltage is constant.
If a high-frequency AC voltage of several MHz or more is used, however, the response of a detection circuit to determine an average voltage is degraded, so that there is a problem of serious measurement errors. With use of a high-frequency voltage, moreover, the slightest variation in the component characteristics of a voltage application circuit is non-negligible, so that the machining voltage inevitably varies depending on the machine. If axis feed control is performed based on such data, the result of machining inevitably varies according to the machine. Consequently, the control based on the average voltage detection involves many problems such that the electrode feed rate must be made constant, thus hindering the improvement in machining accuracy.
To overcome this, International Publication No. 2004/022275 discloses a technique in which a DC voltage is superposed on a high-frequency AC voltage to be applied, and only a low-frequency voltage ingredient of a machining-gap voltage is extracted by means of a low-pass filter. The feed rate of an electrode is controlled according to the change of the extracted voltage ingredient. Since the average voltage inevitably cannot be zero according to this technique, electrolytic corrosion may possibly occur in a workpiece or machine body. Since the low-pass filter is used, moreover, the response is too poor to enable follow-up in case of a sudden change of electric discharge conditions. Further, the flow of the working fluid in the machining gap varies depending on the machining shape. If the concentration of sludge accumulated in the machining gap changes, the machining-gap impedance changes without the change of the discharge conditions, so that the average voltage may sometimes vary. In some cases, therefore, the average voltage may fail to correctly reflect machining conditions, such as the frequency of electric discharge.
The number of discharges per unit time is an electric discharge characteristic value other than the average voltage. Japanese Patent Application Laid-Open No. 2002-254250 discloses a technique in which the electrode feed rate, off time, and working fluid density are controlled by detecting the discharge number per unit time. The discharge number is an index that is more hardly affected by disturbances due to the sludge density, specific resistance of the working fluid, etc., than the average voltage. Japanese Patent Application Laid-Open No. 2010-280046 discloses a technique in which the discharge number is determined and counted during high-frequency AC machining.
Conventionally, however, the discharge number cannot be detected in a high-precision finish machining application, so that the relationship between the discharge number and optimal electrode feed rate control is not yet fully clarified. Even though the discharge number can be counted, therefore, there is no specific means that enables effective use of the discharge number for machining control.